[C++ PATCH] Avoid bogus -Wunused-but-set* with structured binding (PR c++/85952)

[Jakub Jelinek]

On Tue, May 29, 2018 at 10:49:03AM -0400, Jason Merrill wrote:
> >>   auto & [x,y] = a;
> >>
> >> but that should be added to the testcase.
> >
> > It is already there (in baz).
> 
> Well, yes, but in baz a is an S, not an array; I see value and
> reference cases for S, but only value for int array.

Oops, yes, you're right.  Added qux then that tests array initializer
and & qualifier.

Ok with that change?

2018-05-29  Jakub Jelinek  <jakub@redhat.com>

	PR c++/85952
	* init.c (build_aggr_init): For structured binding initialized from
	array call mark_rvalue_use on the initializer.

	* g++.dg/warn/Wunused-var-33.C: New test.

--- gcc/cp/init.c.jj	2018-05-25 14:34:41.000000000 +0200
+++ gcc/cp/init.c	2018-05-28 19:04:10.504063972 +0200
@@ -1678,6 +1678,7 @@ build_aggr_init (tree exp, tree init, in
       if (VAR_P (exp) && DECL_DECOMPOSITION_P (exp))
 	{
 	  from_array = 1;
+	  init = mark_rvalue_use (init);
 	  if (init && DECL_P (init)
 	      && !(flags & LOOKUP_ONLYCONVERTING))
 	    {
--- gcc/testsuite/g++.dg/warn/Wunused-var-33.C.jj	2018-05-28 19:32:00.236440573 +0200
+++ gcc/testsuite/g++.dg/warn/Wunused-var-33.C	2018-05-29 16:52:21.322700629 +0200
@@ -0,0 +1,37 @@
+// PR c++/85952
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wunused-but-set-variable" }
+
+int
+foo ()
+{
+  int a[2] = {1, 2};	// { dg-bogus "set but not used" } */
+  auto [x, y] = a;	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+  return x + y;
+}
+
+struct S { int d, e; };
+
+int
+bar ()
+{
+  S a = {1, 2};
+  auto [x, y] = a;	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+  return x + y;
+}
+
+int
+baz ()
+{
+  S a = {1, 2};
+  auto & [x, y] = a;	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+  return x + y;
+}
+
+int
+qux ()
+{
+  int a[2] = {1, 2};
+  auto & [x, y] = a;	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+  return x + y;
+}

	Jakub